According to a conventional technique, a shared virtual memory is built among plural terminals connected to each other by a network; and a virtual memory space is shared among the plural terminals (see, e.g., Japanese Laid-Open Patent Publication No. H10-254761). According to another disclosed technique, a table that includes data conversion equations according to terminal attribute and application type is prepared for sharing data among different types of terminals; and when a terminal transmits the shared data to another terminal, the terminal refers to the table, converts the shared data, and thereafter, transmits the converted data (see, e.g., Japanese Laid-Open Patent Publication No. H11-149403).
A data backup method has been disclosed that is used to share digital data among plural terminals respectively connected by a network (see, e.g., Japanese Laid-Open Patent Publication No. 2008-15739). According to this technique, a parity is produced by acquiring an exclusive OR of data “A” that is to be backed up and auxiliary data B retained at another terminal; and the parity is arranged in a terminal other than the terminals retaining the data A and the auxiliary data B. Thereby, even when the data A cannot be regenerated, the data A can be restored by acquiring the exclusive OR of the auxiliary data and the parity.
However, according to the techniques of Japanese Laid-Open Patent Publication Nos. H10-254761 and H11-149403, the data cannot be referred to when the connections among the terminals are disconnected consequent to a network fault, etc. and therefore, a problem arises in that the shared virtual memory is destroyed. With the backing up technique according to Japanese Laid-Open Patent Publication No. 2008-15739, the volume of the auxiliary data and the parity data to be transmitted for back up is great; a long time is consumed from an issuance of a write request to completion of all the processes; and therefore, the response performance of the backing up cannot be improved.
According to the techniques above, although the auxiliary data and the parity data are frequently transmitted and received through the network, the communication speed of the network is extremely low compared to the speed of the reading and writing of the memory in the terminal. Therefore, a long time is consumed for communication during the time period from the issuance of the write request to the completion of all the processes. Consequently, the realization of high response performance is difficult. Thus, when the shared virtual memory is destroyed, the restoration thereof consumes a great deal of time.